There are many places in industry where a force actuator must be remotely controlled to exert a mechanical force sufficient to move an object short distances to a prescribed point and then hold the object at this point notwithstanding variations in the return force of the object of the actuator.
The object may, of course, be any variable weight or force, a motor or pump control, a valve control or the like and the term object is used in a broad generic sense.
Electric motors, electric solenoids and hydraulic pressure energized devices have been used in the past for this purpose, all with varying degrees of success.
Electric motors and solenoid actuators heretofore have offered the simplest type of remote control because only electric wires are required between the actuator and the controls, which latter can be either manual or automatic.
Electric motors must act through gear reducers and are consequently bulky, complicated, expensive and have an overly long total response time. They are otherwise desirable because the gear reducer is usually self-locking to retain the object in any adjusted position regardless of variations of the force of the object on the actuator. However, being self-locking, there is no automatic provision to return the object to its original position in the event of a power failure. Electric motor powered actuators are usually energized with pulses of electric power which must be accurately timed to determine the distance that the object is moved. These timers are complicated, expensive and thus undesirable.
Electric solenoids have a limited force capability for a given physical size and a relatively low force to electrical energy ratio, which energy in the form of an analog electrical signal must be continuously supplied. While solenoids are quick to react for a fixed energizing current, they are unable to hold the object at a fixed position when the force of the object on the actuator varies. To accomplish this latter, complex feedback circuitry is required.
Hydraulic pressure energized actuators are ideal for the purpose. They are capable of exerting very large physical forces for a minimum size. They easily hold the object in any adjusted position even with variations in the force of the object on the actuator. However, to remotely control such an actuator has always been a problem. Electric motor or solenoid actuated valves have been employed but with the difficulties above enumerated. Additionally, the actuator was required to have a control rod extending through packing glands in its housing, which rod then moves a member on the inside of the housing to effect an ultimate control of the operating member. Normally, the inside of the housing is at the high hydraulic pressure. In such a device, the packing gland exerts a substantial amount of friction on the control rod thus requiring substantial mechanical forces to move the rod. Furthermore, this friction, particularly with a solenoid actuator, provides a hysteresis loop in the responsive curve. Also, the gland must be constructed to withstand the full hydraulic pressures and oftentimes leaks develop necessitating maintenance. In addition, such leaks are always messy.
Consequently, it has been conventional with hydraulic actuators to pipe the high pressure hydraulic fluid from a pump to a remote control point, through manually or automatically actuated valves and then back to the hydraulic actuator. In such an arrangement, the distance that the hydraulic fluid has to travel can be extensive. The tubing or piping is expensive. Its installation with many bends therein is costly. Oftentimes, fittings or hoses are required to enable articulation of the actuated device relative to its support. When there are a large number of controls, a large number of pipes or tubes must be employed. The cross sectional area of all of these pipes or tubes can become excessive and bulky. Leaks develop both in the tubes, in the fittings and in the controls. Such leaks are expensive to repair and also messy, particularly when the leaks occur in the controls which are normally in a cab occupied by the operator.
Remotely controlled linear servo motors have long been known. They typically consist of an operating member mounted in a housing and an operating shaft extending through the housing. A null type servo valve is mounted inside the operating member and variably connects either pressure or suction chambers to a third variable pressure chamber.
The control element of the servo valve typically has a shaft protruding from the housing which is positioned by various externally energized force means. Since one end of the shaft is at atmospheric pressure, the other end has to be of the same diameter and vented to atmospheric pressure. This involves elaborate sealing arrangements and great difficulty in holding many ports in absolute concentric alignment particularly when they are substantially spaced apart. No alignment is ever perfect and this results in additional friction associated with the servo valve control element. The elaborate sealing also adds substantial friction in moving the element. This results in severe hysteresis in servo valves. The higher the hysteresis, the higher the force capabilities of the energizing means must be since the overall positioning accuracy can only be as accurate as the total hysteresis force divided by the energizing force. This invention overcomes these problems as well as teaching various additional means of reducing hysteresis friction.
U.S. Pat. Nos. 2,930,360 and 3,131,608 disclose electrically controlled servo motors. In the latter, an electromagnet positioned in the pressure control chamber moves a cylindrical control element to open and close pressure inlet and outlet ports (in the housing) to the control pressure chamber and vary the pressure therein. This control element is operatively associated with the operating piston through a tension spring which moves the control element substantially back to its initial position to restore the pressure balance. Such a device has proven commercially impracticable for various reasons, e.g. the electric coil is submerged in the operating fluid and its insulation is subject to deterioration in modern hydraulic fluids and its replacement is difficult. Further, it only works with spring return and the spring must have a fairly low spring rate. For high fluid pressures where large forces can be developed, physically large springs are required. Further there is a substantial hysteresis problem because of friction between the element and the housing.
The present invention overcomes all of the above difficulties and provides an arrangement whereby a fluid pressure actuator or servo motor may be accurately and remotely controlled by means of conventional electric wiring and an analog electrical signal rather than a timed pulse and the electrical signal may be of a relatively low power compared to the actual force delivered by the actuator.
In addition, the present invention provides an actuator or servo motor which has so little hysteresis and is so precise in its displacement for a given value of an electrical signal or other external force means that it can be used to precisely control fluid valves, e.g. flow, pressure or direction valves, and may also be used to control fluid pumps and/or motors both as to speed and/or direction of rotation. In other words, the present invention may be employed not only as a remotely controlled fluid pressure actuator but may be used as an integral part of a remotely controlled valve wherein the fluid pressure for energizing the actuator comes from the pressure of the fluids being handled by the valve.
The present invention is particularly applicable to the control of the relief pressures of pressure relief valves of the differential area type. Such valves have generally included a valve-member-actuating piston slidable in a cylindrical chamber having a low pressure outlet passage at one axial end of the chamber and a high pressure inlet passage through the side of the chamber. The piston has a valve closing member on its end which engages a valve seat surrounding the outlet passage and a cross sectional area greater than the area of the valve seat. High inlet pressure on this larger area, when the differential forces developed by the pressures were such as to exceed internal spring forces, moves the valve piston and the valve member away from its valve seat to permit the flow of fluid through the valve.
In such valves, the piston diameter is of necessity greater than the diameter of the valve seat in order that forces can be developed that will move the piston to the valve open position. As such, the sliding seal for the piston, which is usually in the form of an O-ring, has a substantial circumference in engagement with the wall of the piston chamber and thus has substantial amounts of friction therewith. In addition, the inlet pressures compress such an O-ring in a direction away from the inlet port, and in the case of very high fluid pressures, tend to extrude portions of the O-ring into the clearance between the piston and its chamber wall. With such an arrangement, the high friction of the O-ring is increased further, particularly as the piston moves from the valve open to the valve closed position. This all results in a hysteresis loop in the pressure vs. flow curve, and in the valve tending to bleed excessively at pressures slightly less than the preset pressure. This is undesirable.
Another problem with such valves has been that as the valve member moves away from its piston seat, the flow of fluid past the valve seat exerts a jet action in a direction opposite the opening movement with the result that the fluid flow opposes the opening movement of the valve seat resulting in a sloping or curved pressure vs. flow curve.
A further principal difficulty in such valves has been that the relief pressure is ordinarily varied by varying the compression of an internal spring by devices such as a control shaft extending through the housing wall which were moved either locally and manually or remotely by means of a gear motor to which is applied a variable length electrical pulse. Such gear motors are bulky, expensive and slow in their response time.
The invention is also particularly applicable to the control of fluid flow valves of the differential area piston type wherein fluid under high pressure is received and delivered at a controlled pressure to a fluid energized motor piston or the like notwithstanding variations in the supply pressure or loads on the motor. As such, the valve must be continuously responsive, opening and closing quickly to any changes. Also the valve should be readily controllable from a remote point. Electric motors and solenoids are undesirable for this purpose for reasons discussed above.
Another problem with such valves when the fluid is an oil is that the flow or output pressure varies with the temperature and/or viscosity of the liquid necessitating complicated feedback circuitry for sensing such changes and correcting therefor. Heretofore expensive means have been provided for heating or cooling the liquid to maintain its temperature constant.
The invention is also particularly applicable to the control of two or four way directional control valves. Heretofore, such valves to be remotely controlled required the use of either two pilot valves, one on each end thereof, or a large solenoid capable of exerting a force larger than the pressure forces developed on the side of the valve by the pressures being controlled. Additionally, such valves heretofore have always required a shaft or shafts extending through high pressure seals for the purpose of actuating the valve armature inside.
The invention may also be used to actuate a fluid pressure intensifier which must be remotely controlled.
As far as I am aware, no one has been able to successfully from a commercial standpoint electrically control the position of the actuating member of a fluid actuator or the opening, closing or flow through a fluid valve without a mechanical control rod extending through a high pressure seal and/or using an analog signal of relatively low wattage, e.g. less than ten watts and oftentimes less than five watts. Further, as far as I am aware, no one has ever employed a ported rod surrounded by a sleeve as the control element in a linear servo motor.